Mechanical Basic Interview Question and Answers Part 2

Basic of Mechanical Engineering Interview, viva ,Oral Question and Answers Part 2

Mechanical Engineering Interview Technical questions and answers with explanation for Job interview, competitive examinations and entrance tests.

Mechanical Basic Interview : Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 

31. Maximum combustion temperature in gas turbines is of the order of 1100 to 10°C whereas same is around 00°C in I.C. engine ? Why ?

High temperature in I.C. engine can be tolerated because it lasts for a fraction of second but gas turbines have to face it continuously which metals can’t withstand.

32. Why efficiency of gas turbines is lower compared to I.C. engines ?

In gas turbines, 70% of the output of gas turbine is consumed by compressor. I.C. engines have much lower auxiliary consumption. Further combustion temperature of I.C. engines is much higher compared to gas turbine.

33. What do you understand by timed cylinder lubrication ?

For effective lubrication, lub oil needs to be injected between two piston rings when piston is at bottom of stroke so that piston rides in oi during upward movement. This way lot of lub oil can be saved and used properly.

34. What is IIUCR in relation to petrol engine ?

HUCR is highest useful compression ratio at which the fuel can be used in a specific test engine, under specified operating conditions, without knocking.

35. In some engines glycerine is used in place of water for cooling of engine. Why ?

Glycerine has boiling point of 90°C which increases its heat carrying capacity. Thus weight of coolant gets reduced and smaller radiator can be used.

36. Why consumption of lubricating oil is more in two-stroke cycle petrol engine than four-stroke cycle petrol engine ?

In two-stroke engine lub oil is mixed with petrol and thus some lub oil is blown out through the exhaust valves by scavenging and charging air. There is no such wastage in four stroke petrol engine.

37. As compression ratio increases, thermal n increases. How is thermal n affected by weak and rich mixture strength ?

Thermal n is high for weak mixture and it decreases as mixture strength becomes rich.

38. How engine design needs to be changed to burn lean mixture ?

Engine to burn lean mixture uses high compression ratio and the highly turbulent movement of the charge is produced by the geometry of the combustion chamber.

39. Horse power of I.C. engines can be expressed as RAC rating, SAE rating, or DIN rating. To which countries these standards belong ?

U.K., USA and Germany respectively.

40. What is the use of flash chamber in a vapour compression refrigeration cycle to improve the COP of refrigeration cycle ?

When liquid refrigerant as obtained from condenser is throttled, there are some vapours. These vapours if carried through the evaporator will not contribute to refrigerating effect. Using a flash chamber at some intermediate pressure, the flash vapour at this pressure can be bled off and fed back to the compression process. The throttling process is then carried out in stages. Similarly compression process is also done in two separate compressor stages.

41. Why pistons are usually dished at top ?

Pistons are usually hollowed at top to (i) provide greater spa’e for combustion, (ii) increase surface for flue gases to act upon, and (iii) better distribution of stresses.

42. What is the function of thermostat in cooling system of an engine ?

Thermostat ensures optimum cooling because excessive cooling decreases the overall efficiency. It allows cooling water to go to radiator beyond a predetermined temperature.

43. What are the causes of failure of boiler tubes ?

Boiler tubes, usually are made from carbon steel and are subject to (a) high rates of heat transfer,( b ). bending stresses due to uneven heating, especially at expanded or welded joints into headers or drums, (c) external erosion from burners and flue gas, (d) possible corrosion on the boiler side, and (e) occasional manufacturing defects.

Failure may occur due to following reasons :

• High thermal ratings may lead to rapid failure if the internal fluid flow is reduced for any reason. The resultant overheating leads to a failure by creep, characterised by the bulging of the tube with the eventual development of a longitudinal split. Fatigue cracking due to bending stresses occur. These are associated with change of section and/or weld undercut, where tubes are expanded or welded into headers.
• Failure may arise due to overstressing of a reduced section of metal.
• Sudden failure of the boiler tube due to corrosion arises from embrittlement of the carbon steel due to interaction between atomic hydrogen from the corrosion process and the iron carbide present in the steel.
• Defects in tube manufacture, although far from being a regular occurrence, can be a cause of serious trouble. Lamination in boiler tubes or score marks arising from the cold drawing of tubes, give rise to premature failure and may promote corrosion at these regions.

44. What are the causes of failure of superheater tubes ?

Superheater tubes are subjected to the most severe combination of stress, temperature and corrosive environment. In addition to high-temperature strength, resistance to corrosion is also important. For example, low-alloy ferritic steel such as -1/% Cr, 1% Mo would not be used at metal temperatures above 580°C because of inadequate resistance to corrosion and oxidation over a full service life of 100,000/150,000 hr.

Failures in superheater tubes may arise from :
(a) Prior fabrication history
(b ) Faulty heat treatment
(c) Consequences of welding
(d) Overheating of the tube metal
(e) Gas-side corrosion
(f) Stress corrosion (austenitic steels).

45. Why supercritical boilers use less amount of steel compared to non-supercritical boilers ?

Supercritical boilers do not head heavy drum for separation of steam from mixture of water and steam.

46. Out of electric heater and heat pump, which is economical in operation ?

Heat pump.

47. Which furnace burns low-ash fusion coal and retains most of the coal ash in the slag?

Cyclone furnace.

48. How the thickness of thermal boundary layer and thickness of hydrodynamic boundary layer related ?

Ratio of their thickness = (Prandtl number)-1/3.

49. What is the effect of friction on flow of steam through a nozzle ?

To decrease both mass flow rate and wetness of steam.

50. Why gas turbine power plant needs efficient compressor ?

Because a large portion of turbine work is eaten away by compressor and its inefficiency will affect net power output and cost of generation.

51. Why rockets using liquid hydrogen have higher specific impulse compared to liquid hydrocarbon ?

Liquid hydrogen has higher burning velocity.

52. Why axial flow compressor is preferred for gas turbines for aeroplanes ?

Because it has low frontal area.

53. What is the effect of inter cooling in gas turbines ?

It decreases thermal efficiency but increases net output.

54. Why iso-octane is chosen as reference fuel for S.I. engines and allotted 100 value for its octane number ?

Iso-octane permits highest compression without causing knocking.

55. Why thermal efficiency of I.C. engines is more than that of gas turbine plant ?

In I.C. engine maximum temperature attained is higher than in gas turbine.

56. Which are the reference fuels for knock rating of S.I. engines ?

n-heptane and ISO-octane.

57. When effect of variations in specific heats is considered then how do maximum temperature and pressure vary compared to air standard cycle ?

Temperature increases and pressure decreases.

58. Quantities like pressure, temperature, density, viscosity, etc. are independent of mass. What are these called ?

Intensive properties.

59. The amount of radiation emitted per scm per sec is called …. ?

Emissive power.

60. In convection heat transfer, if heat flux intensity is doubled then temperature difference between solid surface and fluid will ?

Get doubled.

Mechanical Basic Interview : Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 

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